Writing instrument



FIG. 1 is a front perspective view of an ornamental design for a writing instrument according to the present invention;

FIG. 2 is a front elevational view thereof;

FIG. 3 is a rear elevational view thereof;

FIG. 4 is a left side elevational view thereof;

FIG. 5 is a right side elevational view thereof;

FIG. 6 is a bottom plan view thereof; and,

FIG. 7 is a top plan view thereof rotated 180 degrees from FIG. 6.

The pushbutton, clip, barrel, and writing tip in broken lines are included to show features that form no part of the claimed design. 

CLAIM The ornamental design for a writing instrument, as shown and described. 